Firstly, there will be explained cases in which a ship equipped with a radar apparatus is at rest or the distance travelled by a moving ship per unit time can be disregarded.
There have been some conventional radar apparatuses which are provided with three display modes; North up display mode (hereinafter referred to as "NU display mode"), Helm-Perspective display mode (hereinafter referred to as "HP display mode"), and Course-up display mode (hereinafter referred to as "CU display mode"). These three display modes can be selected as desired.
1 With NU display mode, the top of the display screen always represents the north as shown in the FIG. 4. For instance, when a course of ship is 0.degree. (with respect to the north as a reference bearing), the heading line of the ship is located at the position shown by a solid line with an arrow. When the course of ship is changed by 90.degree. clockwise (eastward), the heading line moves to a position shown by a dotted line with an arrow. But, immovable things such as buoys are displayed at the same points regardless of the course of the ship. In other words, displayed images represent a scenery an operator can see from the sky, not from the ship. PA1 2 With HP display mode, the top of the display screen always represents the heading line of the ship as shown in FIG. 5. For instance, a fixed image representing a fixed buoy and the like are located at an A1 position on the line when the course of the ship is adjusted to 0.degree., and the fixed image will move to an A2 position when the course of the ship is changed clockwise by 90.degree. . Thus, the resultant displayed image becomes a scenery an operator can see on the ship. PA1 3 With CU display mode, the top of the display screen always indicates a predetermined scheduled course, as shown in FIG. 6. For instance, when the course of the ship is set to a bearing 10.degree., the top of the display screen will always represent a bearing 10.degree.. The heading line of the ship is indicated as a displayed solid line with an arrow when the ship is advancing along a predetermined scheduled course. However, when the actual proceeding course of the ship is changed in order to avoid yawing and obstacles, the heading line of the ship moves as shown by virtual lines on the screen. But, an image representative of an immovable target is displayed at the same point regardless of changes of the course of the ship. PA1 1 As shown in FIG. 14, RM displays in a manner that the ship (the center of the radial radar sweeps) is fixed at a specific point (at the center in the drawing), and fixed targets are being moved on the screen of an indicator. PA1 2 As shown in FIG. 15, TM displays in manner that the ship (the center of the radial radar sweeps) is moved depending on its speed and proceeding bearing, and immovable targets are fixed.
Referring to FIG. 7, a receiving unit 1 receives, detects and amplifies echo signals resulting from each of respective search pulse signals radiated successively in different directions by means of a rotating radar antenna (not shown) and coming from one of the respective different directions. The analog reception signals resulting from one search pulse signal and produced by the receiving unit 1 are converted to digital signals by means of an A/D converting unit 2 and then stored in a buffer memory 3 in a time sequential fashion. The reception signals (echo data) from the buffer memory 3 are converted to image display data by a write data generating unit 4.
The write data generating unit 4 implements a scan correlation process on the echo signals caught by the radar antenna in order to eliminate unwanted echo signals such as echo signals reflected by the sea surface. This scan correlation process generates data signals for one rotation of the radar antenna by utilizing reception signals obtained during ten rotations of the radar antenna, for instance, as disclosed in patent publication of an unexamined Japanese patent application No. SHO 62-223681. The scan correlation process will be briefly explained. Reception signals received during a first rotation of the radar antenna are compared with receptions signals obtained during a second rotation of the antenna to produce comparison result data in accordance with predetermined relationships. Then, the comparison result data are compared with the reception signals caught during a third rotation of the antenna to produce new comparison result data. Thereafter, the same process is repeatedly performed to remove images due to unstable reflections as sea clutter. Thus, there are utilized to perform the scan correlation present reception signals supplied by the buffer memory 3 and old reception signals which have already been written and stored in the frame memory 6.
The output data produced from the write data generating unit 4 are written into memory elements of the frame memory 6 identified by write addresses generated by the write address generating unit 5. The data written into the frame memory 6 are read out based on read addresses generated by a read address generating unit 7 in compliance with a raster scanning method. The data read out are displayed as images on an indicator such as a CRT which is not shown.
The write address generating unit 5 generates write address signals representative of coordinates defined in Cartesian coordinates (X, Y) based on bearings of the antenna, a selected display mode and an advancing bearing of the own ship, thus performing a conversion from polar coordinates to Cartesian coordinates. This conversion is performed in accordance with the following equations. EQU X=X.sub.C +R sin .THETA., Y=Y.sub.C +R cos .THETA.
where .THETA.=.THETA..sub.A in HU display mode selected; .THETA.=.THETA..sub.A +.THETA..sub.G in NU display mode selected; and .THETA.=.THETA..sub.A +.THETA..sub.G -.THETA..sub.C in CU display mode selected.
X.sub.C, Y.sub.C are addresses representative of a position of the ship in the frame memory; R is distance from the ship; .THETA. is an angle in a sweep direction with respect to Y-axis in the frame memory; .THETA..sub.A is an angle of the antenna with respect to the heading direction of the ship; .THETA..sub.G is an advancing bearing of the ship; and .THETA..sub.C is a selected course.
It should be noted that with the radar apparatus in the foregoing, if a raster scanning method is employed to read out the indication signals to the indicator, the write address generating unit 5 is constructed to perform coordinates conversion from polar coordinates to Cartesian coordinates. But, if a spiral scanning method is employed, it is not required to perform the coordinates conversion in the write address generating unit 5.
With NU display mode or CU display mode selected to display echo signals on an indicator, in cases in which the ship is at rest or the distance travelled per unit time by the moving ship can be neglected, data representative of fixed targets are written into the same memory elements of the frame memory 6 so that the fixed targets are displayed at fixed points on the screen of an indicator, even if data written into the frame memory 6 obtained during one rotation of the antenna are updated every moment. Thus, conventional radar apparatuses are also capable of appropriately performing a scan correlation process and removing undesired waves as sea clutter. However, while HP display mode is selected, the heading line of a ship equipped with a radar apparatus always changes so that data representative of fixed targets obtained during one rotation of the antenna may be written into different memory elements of the frame memory 6 every one rotation of the antenna. As a result, there is brought about a drawback that the scan correlation process cannot be done correctly and thus images representative of fixed targets move on the screen of an indicator every moment.
It should be noted that HP display mode is comparatively often used, since an actual scene observed by an operator directly corresponds to an image displayed on the screen of an indicator.
Further, with conventional radar apparatuses, tracks of targets cannot be displayed, when HP display mode is selected. In order to display tracks of targets, in short, reception signals presently being received are written into the frame memory into which old reception signals have been previously written. With HP display mode selected, when data written into the frame memory are updated every moment, the heading direction of the ship always moves so that reception signals being presently received can not be written into the same memory elements into which old reception signals have been stored. Accordingly, tracks of targets cannot be indicated accurately.
Next, there will be explained a case in which a ship equipped with a radar apparatus is moving and the distance advanced per unit time by the ship cannot be neglected.
A block diagram illustrating the construction of a conventional radar apparatus shown in FIG. 13 is almost the same as the block diagram of a conventional radar apparatus shown in FIG. 7. Differences between the block diagram of the conventional radar apparatus shown in FIG. 7 and the one in FIG. 13 are that signals representative of a position Of the ship are supplied to a write address generating unit 5m and a read address generating unit 7m. The output signals of the write data generating unit 4 are written into the frame memory 6 based on write address signals produced by the write address generating unit 5m. The data signals written into the frame memory 6 are read out based on read address signals produced by the read address generating unit 7m in accordance with a raster scanning method. The write address generating unit 5m converts points at which echo signals are produced defined with respect to the radar antenna in polar coordinates system to coordinates defined in Cartesian coordinates based on the antenna bearing, the advancing course of the ship, the position of the ship and a display mode selected.
It is to be noted that there are two kinds of motions, a relative motion (hereinafter referred to "RM") and a true motion (hereinafter referred to "TM"). RM displays the ship at a fixed position; and TM displays the ship being moved depending on distance travelled by the ship. One of the motions is selected by an operator by means of a mode selecting unit (not shown).
In order to present these displays, the address of memory element of the frame memory 6 corresponding to the center of the radial radar sweeps is fixed or varied when data signals are written into the memory. In other words, the way to fix the center of the radial radar sweeps at an address of memory elements in the frame memory 6 is referred to as RM writing process. The other way to vary the address of memory elements corresponding to the center of the radial radar sweeps is referred to as TM writing process.
With regard to a starting address of memory element during an indication operation, there are two ways in the same way as the foregoing way to identify an address of memory elements corresponding to the center of the radial radar sweeps. One way is to fix the starting address at an address in the memory and is referred, for convenience, to RM reading process, and the other way is to vary the starting address in the memory and is referred to TM reading process.
The following four combinations of the writing processes and the reading process are selected and performed respectively.